The restriction and degradation of foreign DNA in Escherichia coli B occurs through the joint action of Eco B, the Rec B DNase and exonuclease I. Host DNA is protected from Eco B by the methylation of specificity sites by the DNA methylase; host DNA is protected from the rec BC DNase and exonulease I in an unknown manner, but when the DNA is damaged, rec A protein seems to be involved. We plan to study the mechanism of DNA degradation by Eco B and the rec BC enzyme and what factors regulate these enzymes. We also plan to study the interaction of rec A protein with rec BC enzyme and exonuclease I. We hope also to learn more about the significance and properties of a new form of E. coli DNA polymerase I that appears to be induced with rec A, as to study further, both mechanistically and genetically, two endonucleases that we discovered previously as contaminants of Eco B - endonuclease III and endonuclease V. The latter enzymes are implicated with DNA repair. Finally, we plan to study how W. coli DNA polymerase I treats termini produced by these repair enzymes during DNA synthesis.